There are a variety of applications which use a serial transmitter circuit of the type that receives a series of multi-bit words at a parallel input, and then transmits those words serially in an end-to-end manner at a serial output. During the manufacture of an integrated circuit which includes such a transmitter circuit, the transmitter circuit must be tested for proper operation. Where the transmitter circuit will be used in real-world applications that involve high data rates, it is appropriate to test the transmitter circuit at comparable data rates. However, commercial test platforms are typically not capable of operating at comparably high data rates, with the exception of certain high performance test platforms that are prohibitively expensive.
In many applications, the serial communication between opposite ends of a serial communication link involves two-way communications. In other words, both a transmitter circuit and a receiver circuit are provided at each end of the communication link. For applications of this type, it is common to use one integrated circuit at each end of the communications link, where each such integrated circuit contains both a transmitter circuit and a receiver circuit. During testing of such an integrated circuit, the serial output of the transmitter circuit can be coupled to the serial input of the receiver circuit on the same chip, so that the transmitter circuit and receiver circuit can each be operated at the high data rates they will experience in normal operation, under the control of an external test platform which does not need to directly monitor the high-speed serial data stream, and which can thus operate at a substantially slower speed than the serial data stream.
There are other applications, however, where the serial data communications across a serial communications link effectively involve only one-way communications. One example of such an application is a portable computer of the type commonly known as a notebook computer. A notebook computer typically has a case and a lid which are pivotally coupled to each other, the case containing the microprocessor and most other circuitry, and the lid containing a liquid crystal display (LCD). When the microprocessor is executing a state-of-the-art program which includes a graphical user interface (GUI), display data must be transmitted from the microprocessor to the display at a very high rate, but there is no data which needs to be sent from the display back to the microprocessor. Thus, the communications between the microprocessor and the display are one-way communications.
Due to the fact that the lid is pivotally coupled to the case, it is desirable to minimize the number of wires which must be routed from the case to the lid through the hinge connection. Accordingly, it is customary to send this data serially from the microprocessor through the hinge to the display. Since the serial communications are one-way, the microprocessor needs a serial transmitter circuit, and the display needs a serial receiver circuit, but the microprocessor does not need a serial receiver circuit and the display does not need a serial transmitter circuit. Thus, for purposes of normal operational use, the integrated circuit which includes the serial transmitter circuit for the microprocessor does not need to also include a serial receiver circuit. Consequently, it is not possible to use the traditional test technique described above, in which the serial transmitter and receiver on a given integrated circuit are serially coupled to each other for purposes of test. There is thus an issue of how to effectively test such an integrated circuit, which has a serial transmitter, but no serial receiver.
One approach would be to use a high-speed test platform. However, as noted above, they are prohibitively expensive. An alternative approach would be to provide an entire deseralizing receiver circuit in the same integrated circuit chip, solely to permit testing of the serial transmitter circuit using the traditional technique, while using a test platform which is inexpensive and operates at slower rates. However, this can almost double the size of the overall circuitry in the integrated circuit, and thus the physical size of the integrated circuit. Due to the increased complexity and size, the integrated circuit costs more, and is more susceptible to manufacturing defects and mismatches that reduce the effective yield of chips from the manufacturing process.